Examples of today's infrared data communication. devices are remote controllers used in conjunction with most popular household electrical appliances personal computer peripheral equipment using the standard optical space communication elements IrDA1.0 and 1.1.
The remote controller has a transmission rate of about 1kbps and a one way communication path, but it is, characterized by its long transmission distance (over 10 m). On the other hand, a device using the optical space communication elements IrDA1.0 and 1.1 has a short transmission distance of about 1 m, but it is characterized by its transmission rate as great as 9.6 kbps-4 Mbps and its ability of sending a large volume of data bi-directionally.
There will be an increasing need to accelerate the operation speed to improve the transmission rate and upgrade the characteristics, such as sensitivity, to extend the transmission distance for the data communication receiving elements used in these data transmission devices. Further, it is inevitably necessary to lower a power source voltage for the data communication devices and downsize a product package. Thus, new techniques are required to solve the problems arising in accelerating the operation speed and upgrading the sensitivity of these data communication receiving elements.
FIG. 18 is a circuit diagram of a conventional data communication receiving element 101, which basically comprises a photo-receiving section 102 for receiving a light signal, an amplifier circuit 103 for amplifying a received signal, and a waveform shaping circuit 104 for shaping the waveform of the amplified signal.
The photo-receiving section 102 is composed of a photodiode PD101. The amplifier circuit 103 is composed of amplifiers AMP101 and AMP102 in two stages, and a capacitor C101 provided somewhere between the amplifiers AMP101 and AMP102. The amplifier AMP101 in the first stage includes a feedback resistor R101. The waveform shaping circuit 104 is composed of a peak hold circuit PH101, a voltage dividing circuit BC101, a comparator CMP101, and an output circuit OC101. Further, the voltage dividing circuit BC101 is composed of resistors R102 and R103, and the output circuit OC101 is composed of an output transistor Qo101 and a pull-up resistor R104.
The photodiode PD101 converts a received infrared light signal to an electrical signal (current), which is amplified by the amplifier circuit 103. Here, the amplifier AMP101 amplifies the electrical signal by converting an output current from the photodiode PD101 to a voltage. Also, only a signal component in an output voltage from the amplifier AMP101 is allowed to pass through the capacitor C101 and amplified by the amplifier AMP102 in the following stage.
An output voltage from the amplifier AMP102 is inputted to the peak hold circuit PH101 and the non-inverting input terminal of the comparator CMP101. An output voltage from the peak hold circuit PH101 is divided to a detection level in a predetermined ratio by the resistors R102 and R103 of the voltage dividing circuit BC101 first, and thence inputted to an inverting input terminal of the comparator CMP101.
The comparator CMP101 compares the output voltage from the amplifier AMP102 with the detection level, and outputs a voltage shaped into substantially a square pulse. The output transistor Qo101 of the output circuit OC101 switches ON/OFF in response to an output voltage from the comparator CMP101. When the output transistor Qo101 stays ON, it outputs a low-level output voltage Vo101 to an output terminal OUT101. On the other hand, when the output transistor Qo101 stays OFF, it outputs a, high-level output voltage Vo101 to the output terminal OUT101. At this point, the output voltage Vo101 is shaped into a substantially perfect square pulse.
FIGS. 19(a) through 19(c) show operation waveforms at each of the points L, M, N, 0 and P (output terminal OUT101) shown in FIG. 18. A waveform a of FIG. 19(a) represents a transmitted infrared signal (point L). In FIG. 19(b), a waveform b represents an output voltage (point M) from the amplifier circuit 103, a waveform c represents an output voltage (point N) from the peak hold circuit PH10, and a waveform d represents a detection level (point O) obtained by dividing the waveform c by means of the resistors R102 and R103 of the voltage dividing circuit BC101. A waveform e of FIG. 19(c) represents the output voltage Vo101 (point P) outputted to the output terminal OUT101, which is obtained by comparing the waveform b with the waveform d by means of the comparator CMP101, and then shaping the comparison result to a waveform by means of the comparator CMP101 and output circuit OC101.
On the other hand, as illustrated in FIG. 20, the conventional high-speed and highly-sensitive data. communication receiving element 101 is downsized by molding a photodiode chip 105 and a receiving IC chip 106 into the same resin. The data communication receiving element 101 includes broadband amplifiers AMP101 and AMP102 so as to operate at a high speed, and is able to detect a very minor input signal due to its high sensitivity.
However, in the conventional data communication receiving element 101 of FIG. 20, a coupling capacity cfb101 is generated across the input end including a wire WR101 and an input terminal IN.sub.IC 101, and the output end including a wire WR102 and the output terminal OUT101.
Thus, a feedback current ifb101 is generated by the coupling capacity cfb101 and a change in the output; voltage Vo101 on a proportional basis with respect to a lapse of time, and flows toward the input end from the output end. The feedback current ifb101 can be expressed as: ifb101=cfb101.times.dVo101/dt, and becomes a noise component in the receiver, thereby deteriorating an S/N ratio given by a ratio of the signal component to the noise component (signal component/noise component). In particular, since the output voltage Vo101 is substantially a square pulse, its rising and falling edges are very sharp. Hence, dVo101/dt takes a very large value, and the feedback current ifb101 increases accordingly.
The above phenomenon makes the transmission distance shorter, and moreover, causes an output of an unwanted error pulse or an oscillation, thereby posing a problem that the performance of the receiving system is degraded.
Also, since two comparators CMP102 and CMP103 are provided in the following stage of the comparator CMP101 as shown in FIG. 21, a generally adopted method allows the selection of either the positive-phase output voltage Vo101 or the anti-phase output voltage Vo101', whichever, is preferable.
However, in this method, when the load is connected to the positive-phase output terminal alone, the waveform of the positive-phase output voltage Vo101 varies with the distortion of the waveform caused by the connection with the load, but the waveform of the anti-phase output voltage Vo101' remains intact because it is not affected by the load.
Consequently, the waveform of the positive-phase output voltage Vo101 differs from the waveform of the anti-phase output voltage Vo101', which makes it difficult to obtain the anti-phase output voltage Vo101' having exactly the same waveform with respect to the positive-phase output voltage Vo101.